Method forithe continuous produc-



Oct. 26, 1954 K. KAHR I 2,692,878

METHOD FOR THE CONTINUOUS PRODUCTION OF PURE LACTAMS Filed July 10 1951 ATTORNEY formed in the Beckmann process.

, for the production of polyami solution. ties dissolve in t .solvents, which have to be used for thedissolution Patented Oct. 26, 1954 METHOD. EQRITHE ooNrLNuQUSJmQDUC- ,rrou .or PURE LACTAMS Kurt Kahr, Ems; Switzerland,

ass ignor to Inventa A. j G. fur Forsehung un'd Patentverwertung,

-Zunich, -Switzerland .re ueuon -Ju ile 1951 seri teeeoe This invention relates toa method for thecontinuousv production .of ure lactams and moreparticularly to amethod v fo1t the extraction of flpure lactams from raw lactar s. v

e e f t -eminee rben e a id steps-e as starting material for the production of polyamides.

I As known in the art, raw lactams are formed by ck a e ne' mente th erres id i cyclic oximes.

h y are ,o einedbr neu r lizatie o ther action mixtures of the. Beckmann rearrangement with aqueous solutions of neutralizing. agents.

The raw lactams, though-waterj-soluble, are thereby separated and salted out of .the concentrated: salt solutions as lactam oils which are almost free of salt and contain about 30,to 40.% water. 7 J

In addition to thellactams also about 3 to 8 92 of icy-products of: basic as well as acid nature are These, are impuritieswhich haveto be rem vedQfrOm thelactamsbecause due-to these impurities; the raw lactams are ofa yellow to brown color.

The isolation of the lactams hitherto took place by. extraction or distillation. flo obtain completely colorless pure lactams,-.such as lare used es, it. was heretofore definitely necessary that the isolated and t pro-cleaned lactams be; purified by. atieast ,,o ne

further distillation. .Now, less lossesoccurin the extraction of the lactams fromtheraw. laotams than in the distillationprocess. Due ,to-the high temperature necessary inthe latterprocess decompositions are unavoidable even if theprocess is performed in a vacuum.

Only such solvents have been used, heretofore for the extraction which dissolve morethan 50% of the lactams at roomtemperature, Extraction means of this kind are in: particular low boiling point chlorinated hydrocarbons, ,for instance chloroform, ethylenchloride.,methylene, chloride, and the like. In order. to recover the lactams in maximum quantitiesfrozn the raw lactams, alone or together with; the concentrated-salt solutions present .in the mixture, it was, absolutely necessary to use extraction m eans of high solubility. Hereby, therawlactams dissolve; completely in small quantities. of the solvents. whereby strongly colored and concentrated lactam, solutions are obtained. Almost all; of sthevvater. of the raw lactams separates as awdiluted aqueous laotam Te e h w ththe et m e pu ee eid reb e rn r ert e st n t .of t elaeta .Tl tesu tiee eluti nee m ch ve by fires W Q teb own 919 removal of theimpurities ean only be performed orless strongly colored by the impurities and present atter, removal of the solvents lactams of explained before, the

by a distillation process in which complete removal of theimpurities results in a considerable leese el b eei eethe, m ur ti are al of t same. eili s points. as t e e t .In urifi ati njbv di t ll t additions of acidsor al kaline solutions in amounts upto'5% acid esters beiore the distillation of the lactams.

By these additives, howeyer, t he yield of distilla- .iiQn is stronglyreduced. I 'Ifhe additives, which are added in excess in relation to the quantity of the changed impfu ties remain jbehind il at e residuev", if e yi l s, of u e .tams an ountto about 90%;

lactams, the yields are even considerably lower.

. mannentegether with le tems i ithe t W .impmii ie ihavea deeempesins fi 011 t ,tams. at the high temperatures of distillation.

Apart of these decomposition products. vapor- ,izes withl ,thelactams so that absolute purity of the f la ctams oannot be achieved.

The major portion of the pr ducts of decomposition and the in the dis- 7 lactams obtained in this manner from the pre purified lacreferred to the raw lf solvent s with lower dissolving capacities for lacta nsare used for the extraction of the raw lac- .tams,large-quantities of solventare required and dilute lactain solutions areobtained. It is uneconomical to extract the raw lactams with these .g lvente-enln s. dur n the. senie tp th s laet extraction, the water increases due tothe re m oval ,of ,the lactams and the solubility of the hich is almost free of salt, s re t tha in th e ra tion ean lf the. raw lactamsareeigtracted in a similar the concentrated salt solut ns resent by. t r redu ti n.t e v l em V aredehydratd and salted out, whereby three layv...ers are, formed, i.. e..1a ctam solvent, undissolved 'e -leetan se traction, looweYfil'... Will .becon1e. .difficult in. the

solution. A continuous expres neee e d th e -J m- I It. is the object ot this invention to overcome thedis restae s aperieeeed'h re preside ape en e fi eti .steee e for ei eue p ed etie ae l; Pur .leei m lb tr tinuous process, if the lactams in diluted aqueous raw lactam solution satiated with neutral salts are subjected in a plurality of stages to a preliminary extraction with selective organic solvents according to the counterflow principle.

Thereupon, the already considerably purified lactam solutions are furthermore extracted with oxidizing or reducing acid or base binding aqueous solutions or they are adsorbed with solid materials possessing these qualities,whereafter the solvent is finally removed.

Suitable extractors usable according to the invention are such as have almost no solvent power for water, and which thus, at 20 C. dissolve less than 0.1% water. They should dissolve lactams not too well, i. e., at 20 C. not more than 30%. The boiling points of the extraction means must not exceed 100 C., as they are otherwise hard to separate from the lactams by distillation. They may consist of mixtures of several solvents, preferably of azeotropic boiling solvents.

A suitable mixture of this kind consists, for instance, of 55% benzole and 45% cyclohexane. This mixture boils at 775 C. (Lecat Azeotropiame 1918).

In the preliminary extraction, it is advantageous not to make the concentration of the lactam in the extraction means too high, preferably not over 15%, as below this lactam concentration but little water purities are taken up in proportion to the concentration of the lactam. By using about 4. to times as much solvent as raw lactams, the lactam concentration mentioned can easily be maintained. It is more advantageous to employ a solvent or azeotrope boiling mixture, in which the lactam concentration during extraction cannot rise about Thus, for instance, e-caprolactam in the azeotrope of benzole and cyclohexane has, at C., a concentration of about 12% in the extraction means. By using higher or lower temperatures in the preliminary extraction the lactam concentration in the extraction means increases or decreases. The adjustment of a definite lactam concentration may thus be corrected via the temperature within a certain range.

The advantage of solvents with lower dissolving power for lactams in the preliminary extraction is that only slight quantities of impurities get into the solution. After the preliminary extraction, the impurities, in particular the dyes, which otherwise dissolve in the solvents which dissolve lactams well, separate out as flakes in the separating layer. The lactam solutions resulting from the preliminary extraction contain only slight impurities and are therefore hardly colored. According to the present invention, the lactams are subjected to preliminary extraction out of dilute, aqueous, raw lactam solution, saturated with neutral salts. The adjustment of the salt concentration is done by adding concentrated salt solution as obtained in the production of the raw lactams. The water for dilution'originates from the extracted raw lactam solutions, the saturation concentrations for the salts being high with low lactam concentration and low with high lactam concentration. Thus, the saturation concentration, for instance for ammonium sulphate with concentrated, raw e-caprolactam with abt. 66% lactam content, is around 1%, and with dilute raw caprolactam with 33% lactam content, about 8%, and finally, with the excessively diluted raw lactam with a 1.5% caprolactam content, around ammonium sulphate. If concenand small quantities of the im trated salt solution is added to a dilute raw lactam solution which is not yet saturated with salt, the lactam solution absorbs salt to the saturation point, the remaining salt solution separating from the dilute raw lactam solution.

When a dilute, aqueous raw lactam solution saturated with salt is extracted, one obtains a lactam concentration in the extraction means which is equally as high as when extracting concentrated salt saturated raw lactam solutions.

The advantage of the extraction method described is that, when maintaining salt saturation in the dilute raw lactam solution, no intermediary layer of concentrated raw lactam separates out which would complicate the continuous preliminary extraction and even make impossible the use of tower extraction. The preliminary extraction is advantageously carried out in counterflow in such a manner that fresh raw lactam solution, which, in the first extraction stage is diluted to a certain concentration and extracted with a solvent enriched with lactam, is directed in the following extraction stages, under continuous dilution of the raw lactam solution, toward a solvent with lower lactam content. At the end of the preliminary extraction, fresh extraction means flows counter to an almost lactamfree, aqueous salt solution. In the individual preliminary extraction stages, the current is in the main determined by the quantity of the solvent, which amounts to more than four times the volume of the raw lactams. When using an extraction means which has a lower specific gravity than the most dilute raw lactam solutions, the main current in the extraction stages therefore goes from the bottom toward the top. The raw lactams are therefor advantageously introduced in the lower part of the preliminary extraction stages and carried upward by the current, whereby they are extracted. The concentrated salt solutions flowing into the preliminary extraction stages to maintain salt saturation are added to the upper part and fall downward due to its higher specific gravity. As the partly extracted raw lactam solutions rise to the top in the preliminary extraction stages, the salt solution flowing in the opposite direction effects constant saturation. In the last, preliminary extraction stage, which contains little lactam, the addition of salt solution for saturation is considerable, so that its downward flow has an influence. In this stage dilute raw lactam solutions are advantageously introduced in the upper or middle part of the preliminary extraction stage.

A great advantage of the preliminary extraction process described is the considerably facilitated separation of impurities. The solubility of the impurities is greater in the raw lactams than in the salt solution, into which they are, for the most part, to enter in the course of the extraction. On the one hand, this is facilitated by the fact that the impurities are taken up by the aqueous solution very gradually. Furthermore, according to the process as invented, the preextracted lactam solution is drawn off at the point where the lactam-richest aqueous raw lactam solution is located. In this manner, the impurities are, to a large extent, retained in the aqueous raw lactam solution. A part of the impurities separates out in the form of flakes where the raw lactam solution has the least lactam and may be removed from the separating layer by drawing off. The lactam solutions obtained after this preliminary extraction contain considerably less impurities than the lactam solutions obmanager-7s tained according tofthefextraction processes used hitherto. vThese slightimpurities-must, however, be removed .if the llactams are .to '-.be further processed into polyamides.

According .to the invention, this is achieved by a subsequent extraction with anaqueous solution or by adsorption to anion exchangers in the presence or water.

"In this manner colorlesslactamsolutions are obtained, which, after removal of the solvent, for instance by vacuum distillation, resultin completely colorless, unchanging pure lactams.

As aqueous solutions 'for secondary extraction, aqueous solutions of oxidizingmaterials in minimum concentration may be used. Effective as oxidation means are, for instance, already an 12/500 potassium permanganate solution and/or a 0.05% hydrogen peroxide solution, whereby the affected impurities remain "in the aqueous solution. But alsoalkali solutions as, for instance, weak solutions of 'sodalye, sodium carbonate, or calcium hydroxide greatly contribute, 'in the secondary extraction, to the removal of the last traces of impurities by binding the impurities.

Advantageously, solutions with apI-I value between 7 and 12 are employed.

The aqueous solutions serving for secondary extraction contain more or less lactam, according to the lactam concentration in the extraction means. For instance, thelactam concentration in the aqueous solution amounts to about 60% when 12% ecapro-lactamis present in the azeotrope benzole cyclohexane.

The secondary extraction may becarried out with oxidizing solutions in analkaline medium. It may'be conducted in one or morestages. If, for instance, two stagesare used, the two stages may be conducted either only in oxidizing or alkaline solutions, or in a combination of these solutions.

The surprising cleansing eifectofthe aqueous solutions with such slight concentration of additives is only possible because the .preliminary extraction removes the impurities from the lactam solutions with theexceptionofa slight residue. The aqueous solutions used .for secondary extraction, for instance .in the towerextraction process, must be continuously replenished. Hereby, a replacement will be sufficient of, for instance, 1% by volume of a lactam-free, aqueous solution of the additives, referred toa 12% pre-extracted caprolactam solution. (In the hitherto practiced purification of the. lactams by distillation, 100 to 1,000 times the quantity of additives is required.)

To obtain completely purela'ctarns; it is necessary to use only extremely dilute solutions as otherwise the lactam solutions with their even small water contentwould carryalong the dissolved materials, so that they increase in concentration in the lactams.

Advantageously, only such materials are used in the secondary extraction which are not dissolvedout by the lactam-solutions and whichdo not chemically alterthe lactams. Purely inorganic substances are "particularlysuited 'for this purpose. The "secondary extraction is advantageously carried out at room temperature, directly after the preliminary extractionand is also carried out as a continuous process. .The aqueous extraction solutions used up in the secondary extraction may, without difficulty, be returned continuously to .thepreliminary extraction as raw-lactams.

Ion exchangers-may s'BilSOLTbSFalISEdEBSL'fldSOIiJfiOH .means.

In the accompanyingdrawingardiagramishows schematically rhow to -carry outvaspreferre'dembodiment of the invention, i. ':e. the; continuous extraction 10f .the raw lactams in 1a .three-Lsta'ge preliminary extraction'andia singlez'stage secondary extraction.

The preliminary extraction:plantaisirepresented by the towers I, 2, and 3.;-1.the secondaryaextraction is .performedximtowerfl.

The towers are :arranged int-steps, name .below the other, in order .athat :thELSOlVQIIt scan :run through all the towers vby..gravity. Each tower has several stirring zones, on, a, bounded by perforated plates which are SeparatedifrOm each other by a quieting'zoneb. "=The top-an'd bottom parts of the extractionfltowersrarezalso quieting zones 1), b. The separating layers of extraction means and lactam solutionsare maintainedin the upper part .of the :towers. -lhexraw lactam enters the lowest stirring .zzoneof tower :3- at 5. Into the same stirring izone, lactam-enriched solvent from tower 2 fio-wsat I6. .At' 'l i8, and 9 various quantities of concentrated .salt :solutions .flow .into the top stirring zones l of the preliminary extraction tOWBISL-tO adjust the saturation concentration. IFrom itower 3 the solvent richest in'lactam overflows into the :secondary extraction 'tower 4.

The diluted raw lactam =solution, rich in lactam, is drawn-i0ff ia't rthe bottom of tower 3 and is introduced at I 0 into-l-th'e bottom stirring zone of tower 2. Into .thesame stirring zone of tower 2, a solvent with a low lactam content enters at l i fromtower l andleaves-towerl at the top enriched inlactam. Fr-om thebottom of tower 2, a low lactam-contentsaltsolution flows off to one of .the top onmiddle stirring zones of tower i at i2. Fresh solvent enters the "bottom stirring zoneof tower I '-at |"3 -and1-overflows from the top of tower l with .little "dissolved lactam. Fully extracted, concentrated, lactamfree salt solution flows from the bottom of tower at I 4, containing 'the'impurities in soultion. The flakyimpurities separating out are *continuously drawn ofi at T5.

In the secondary extraction tower 4, themeextracted lactam solution enters at H5. The last impurities are extracted in this tower by means of aqueous solutions of additives .with.lactam content. Finally, the entirely .pure, colorless lactam solutionflows oiT :at l l and is freed of solvent by evaporation.

The lactams which remain are completely colorless, unaffected-by light 'or air, and of great purity. The extraction yields amount to more than 98% referred to the lactams contained in the raw lactams.

Theparts mentioned:inithesfollowinguexamples are partsby volume.

Example 11 500 parts of rawe-caprolactam oil flow hourly at 15 G. into .an extraction, plant, as shown in the drawing and described above. Incounterfiow 2500 parts of a solvent mixture of 5.5% ;benzole and 45% cyclohexane are .put.throug-h the,-.preliminary extraction equipment rhourly 1at=t-he same temperature. parts ;::of ca 40% vammonium sulphate solution'enter the third tower, 345 parts of this ammonium sulphate solution enter the second tower and 690 parts of the same enter the first tower hourly at 15 C. Hourly about1300 parts of lactam-free concentrated ammonium sulphate solution, With a dark colora- ,tion due to the dissolved impurities, are drawn I Example 2 Into the extraction plant described above and shown in-the drawing, 500 parts of raw g-enantholactam oil and 2500 parts of a mixture of 55% benzole and 45 cyclohexane flow hourly at 35 C. Into the third tower flow 155 parts of aca. 30% sodium sulphate solution, into the second tower 465 parts of this sodium sulphate solution, and into the first tower 930 parts of the same, all hourly and at 35 C. From the preliminary extraction plant abt. 1700 parts of a concentrated sodium sulphate solution are drawn ofi hourly at 35 C. Into the secondary extraction tower there fiow hourly at 20 0. about 2850 parts of an almost colorless 14% enantholactam solution and 30 parts of an n/100 soda lye which contains 0.5% sodium hydrosulfite in solution. After evaporation of the solvent 9, pure white g-enantholactam is obtained, the yield being 98.5%.

Example 3 500 parts of raw e-caprolactam are preliminarily extracted in the manner indicated in Example 1. Into the first tower of a two-tower secondary extraction plant 2800 parts of a 13% ecaprolactam solution and 28 parts of a cold saturated filtered calcium hydroxide solution containing 0.2% hydrogen peroxide flow hourly. The lactam solution then flows into the second tower where likewise 10 parts of a similar calcium hydroxide solution flow in hourly. After elimination of the solvent there is obtained a snow-white light and air-proof caprolactam, the yield being 99%.

Example 4 500 parts of raw e-caprolactam are extracted in the preliminary and secondary extraction plant described in Example 3. Into the second tower of a secondary extraction plant, however, 14 parts of a saturated aqueous calcium sulfite solution are added hourly. After the elimination of the solvent, e-caprolactam is obtained in the same quality and yield.

Example 5 500 parts of raw'e-caprolactam are pre-extracted as in Example 1. Into the first tower of the two-stage secondary extraction, 25 parts of a solution, which is 11/50 with reference to sulfuric acid and n/250 with reference to potassium permanganate, flow hourly. Into the second tower, parts of a saturated, aqueous calcium hydroxide solution, flow hourly. After passing through these two stages, the lactam solution is evaporated and gives a perfect E-caprolactam, the yield being 98.5%.

Example 6 500 parts of raw e-caprolactam are pre-extracted as in Example 1. The secondary extraction plant consists of two towers, of which the first is filled with 200 parts of synthetic resin cation exchanger and the second with 200 parts of synthetic resin anion exchanger. The lactam solution, having passed through these towers, gives, after elimination of the solvent, 2. pure perfect e-caprolactam, the yield being 98%.

While I have described preferred embodiments of my invention, it will be understood that I do not limit myself in any particular.

Obviously, various modifications of my extraction process may be made within the scope of the following claims.

- I claim:

1. The process for continuous production of pure lactams from raw lactams obtained by the Beckmann rearrangement of cyclic oximes, which comprises saturating diluted aqueous raw lactam solutions with neutral salts, subjecting said saturated solutions to a preliminary extraction with unsubstituted hydrocarbon solvents boiling below 100 C. and dissolving at 20 0. not more than 0.1% water and up to 30% of lactams, in a plurality of stages on the counterfiow principle, subjecting the resulting considerably purified solutions of the lactams to a second purifying treatment with inorganic acid-binding substances in the presence of water at a pH from 7-12 in at least one stage and subsequently eliminating the solvent.

2. The process set forth in claim 1, wherein benzene is used in mixture with saturated hydrocarbon solvents.

3. The process set forth in claim 1, wherein an azeotropic boiling solvent mixture of benzenecyclohexane is used in the preliminary extraction.

4. The process set forth in claim 1, wherein more than four times the volume of the organic solvents is used compared to the volume of raw lactams.

5. The process set forth in claim 1, wherein in the preliminary extraction the concentracted neutral salt solution obtained in the production of the raw lactams is used to achieve salt saturation.

6. The process set forth in claim 1, wherein the concentrated salt solution used for saturation is introduced in counterflow to the dilute raw lactam solution and to the lactam-containing solvent.

7. The process set forth in claim 1, wherein the subsequent purification of the solution resulting from the first extraction is performed with addition of peroxide compounds.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,221,369. Cass Nov. 12, 1940 2,234,566 Lozier et a1 Mar. 11, 1941 2,313,026 Schlack Mar. 2, 1943 2,341,907 Cheetham et a1. Feb. 15, 1944 2,423,200 Moncrieff et al July 1, 1947 2,462,008 Snider et al Feb. 15, 1949 2,462,009 Morris et al Feb. 15, 1949 FOREIGN PATENTS Number Country Date 896,883 France Mar. 6, 1945 

1. THE PROCESS FOR CONTINUOUS PRODUCTION OF PURE LACTAMS FROM RAW LACTAMS OBTAINED BY THE BECKMANN REARRANGEMENT OF CYCLIC OXIMES, WHICH COMPRISES SATURATING DILUTED AQUEOUS RAW LACTAM SOLUTION WITH NEUTRAL SALTS, SUBJECTING SAID SATURATED SOLUTIONS TO A PRELIMINARY EXTRACTION WITH UNSUBSTITUTED HYDROCARBON SOLVENTS BOILING BELOW 100* C. AND DISSOLVING AT 20* C. NOT MORE THAN 0.1% WATER AND UP TO 30% OF LACTAMS, IN A PLURALITY OF STAGES ON THE COUNTERFLOW PRINCIPAL, SUBJECTING THE RESULTING CONSIDERABLY PURIFIED SOLUTIONS OF THE LACTAMS TO A SECOND PURIFYING TREATMENT WITH INORGANIC ACID-BINDING SUBSTANCES IN THE PRESENCE OF WATER AT A PH FROM 7-12 IN AT LEAST ONE STAGE AND SUBSEQUENTLY ELIMINATING THE SOLVENT. 